Detailed modelling of MMA dispersion polymerization in supercritical carbon dioxide

A comprehensive model of dispersion polymerization in supercritical media is presented. With respect to previous ones reported in the literature, this model accounts in detail for the interphase mass transport of the active polymer chains and specifically for its dependence on the chain length. All the model parameters have been evaluated from independent literature sources in the case of poly(methyl methacrylate) dispersion polymerization in supercritical carbon dioxide. The corresponding a priori predictions of the model have been validated with various sets of experimental data, covering a rather wide range of operating conditions, in terms of conversion, pressure and molecular weight distribution as a function of time. It has been found that the interphase mass transport of active chains plays a significant role in determining the number of reaction loci and consequently the final polymer properties and, in particular, the molecular weight distribution.     

水溶性单体分散聚合的研究进展

综述了丙烯酰胺等六类水溶性单体分散聚合的研究现状。     

Solvent effect on TEMPO-mediated living free radical dispersion polymerization of styrene

Living free radical dispersion polymerization of styrene in the presence of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 2,2-azobisisobutyronitrile (AIBN) was conducted in various glycol media having different solubility parameters. Genuine heterogeneous polymerization nature was observed from the initial stage of the reaction. The solubility parameter of the media was found to significantly influence the living characteristics of the polymerization including the polymerization kinetics and the molecular weight evolution at a fixed concentration of TEMPO. A grafting of stabilizer molecules onto monomer and a possible partitioning of TEMPO between the continuous medium and particle phase were postulated for the deviation of the experimentally obtained molecular weight from the calculated value. For a poor medium for styrene, such as diethylene glycol, the living nature was achieved by increasing the amount of TEMPO. The polystyrene (PS) microsphere was well obtained in all tested glycol media and the average size was increased with enhanced affinity of the media to styrene and with decreasing concentration of TEMPO.     

Photoinitiated dispersion polymerization using polyurethane based macrophotoinitiator as stabilizer and photoinitiator

Polyurethane based macrophotoinitiator (PU-PI) had been designed and synthesized, and applied to photoinitiated dispersion polymerization of methyl methacrylate as both photoinitiator and stabilizer, with ethanol/water mixture as reaction medium. Monomer conversion over 90% was achieved within 25 min of UV irradiation at room temperature, and monodisperse PMMA microspheres were obtained. The structure of the microspheres had been analyzed by XPS, showing that about 50% of surface of the microspheres were covered with the stabilizer. PU-PI effectively stabilized the polymeric particles in photoinitiated dispersion polymerization due to the unique stabilization process. The size and size distribution of the microspheres became insensitive to the reaction condition such as stabilizer/initiator concentration, initial monomer concentration and reaction medium. The size of the microspheres obtained changed in the range from 0.88 μm to 1.06 μm at different reaction condition, with polydispersity index as low as 1.011. The research may provide a quick and facile approach to prepare monodisperse microspheres with tailored functional surface.     

The effect of polystyrene-block-poly(4-vinylpyridine) prepared by a RAFT method in the dispersion polymerization of MMA

The dispersion polymerization of methyl methacrylate (MMA) has been carried out using polystyrene-block-poly(4-vinylpyridine) copolymer [P(S-b-4VP)], which was prepared by a reversible addition-fragmentation chain transfer (RAFT) method, as a steric stabilizer in an alcohol media. The stable polymer particles were obtained when the block copolymer concentrations increased from 1 to 10 wt% relative to the monomer and the average particle sizes decreased from 5.3 to 3.4 μm with the increasing concentration of the block copolymer. In particular, the incorporation of 2 wt% polystyrene-block-poly(4-vinylpyridine) produced 4.3 μm of monodisperse PMMA particles with 2.14% of C_v. Thus, the P(S-b-4VP) block copolymer prepared by the RAFT method is working not only as a steric stabilizer, but also in providing monodisperse micron-sized PMMA particles.     

Monodisperse, micrometer-sized low molar mass polystyrene particles by two-stage dispersion polymerization

Free-radical dispersion polymerization of styrene was carried out in ethanol and in ethanol-water mixtures in the absence and presence of carbon tetrabromide (CBr₄) as a chain transfer agent. When CBr₄ was present at the onset of the reaction, particles with a broad size distribution were obtained. If, however, the addition of CBr₄ was delayed ca 1 h, so that the particle nucleation step was complete, then 1-2 wt% chain transfer agent, dissolved in monomer plus solvent, could be added to the reaction without a deleterious affect on particle formation. The particle size and size distribution was essentially identical to that obtained in the absence of CBr₄. When more CBr₄ was added, other problems arose. These problems appeared to be due to solubility of low molar mass polymer in the reaction medium. They could be overcome by running the reaction in ethanol-water mixtures (e.g. 5 wt% water) to decrease the solubility of oligo-styrene at 70 °C, the reaction temperature. In this way, monodisperse particles could be prepared in the presence of 3 wt% CBr₄ based upon total styrene, consisting of polymers with M_n=7060, M_w/M_n=2.4.     

甲基丙烯酸β-羟乙酯分散聚合过程中粒径变化研究

采用十二烷基硫酸钠和聚乙烯醇为分散剂,偶氮二异丁腈为引发剂,用激光散射粒度分布仪对甲基丙烯酸β-羟乙酯（HEMA）分散均聚合过程中粒径变化进行了分析。结果表明聚合物平均粒径起始变化不大,而后随着反应时间的增加大幅下降,说明在聚合过程中大的粒子凝聚,有新的粒子产生;用强度-粒径分布曲线描述聚合过程中粒径变化表明在HEMA分散聚合过程中有周期性成核现象发生,不断有新粒子产生,直到单体聚合完毕。     

Macromonomers having different molecular weights of polyethylene glycol and end group functionalities in dispersion polymerization of styrene

Bifunctional polyurethane-based macromonomers (BPUM) were synthesized from various molecular weights of polyethylene glycol (PEG) and hexamethylene diisocyanate (HDI) with different end group functionalities. The synthesized BPUMs were applied to the dispersion polymerization of styrene in ethanol. Structures of various macromonomers and PS particles were investigated using ¹H NMR. Molecular weights of PEG 1000, 2000, 4000, and 8000 were able to provide stable particles of polystyrene (PS), whereas PEG 400 was not big enough to form stable particles. Although the final conversion was almost constant, average size of the PS microspheres increased with PEG molecular weights. The inverse relationship between the size of particles and molecular weight of PS was observed. In particular, the particle size based on the monomers derived with acrylamide was larger than acrylate derivative monomers. As the molecular weight of PEG increased, the thermal stability was enhanced and leveled off. The grafting ratio of the PS calculated from ¹H NMR spectra exponentially decreased from 1.05 to 0.18 mol% with the increased molecular weight of PEG. Thus, present study suggests that the bifunctional macromonomers act not only as reactive stabilizers, but also as grafting agents in dispersion polymerization.     

Effect of stabilizer concentration and controller structure and composition on polymerization rate and molecular weight development in RAFT polymerization of styrene in supercritical carbon dioxide

An experimental study on the reversible addition-fragmentation chain transfer (RAFT) polymerization of styrene in supercritical carbon dioxide is presented. A 38 mL, high-pressure view cell with two frontal and two lateral sapphire windows was used as reactor. Poly(styrene-block-dimethylsiloxane) was used as stabilizer. The performance as RAFT controllers of S-thiobenzoyl thioglycolic acid, methyl naphthalene dithiobenzoate, 4-methyl allyl dithiobenzoate, and benzyl-N,N-dimethyldithiocarbamate was compared. The effect of stabilizer concentration and controller structure and concentration on polymerization rate and molecular weight development was analyzed. Good performance was obtained with the first three controllers, although simultaneous high polymerization rates and low polydispersities were not possible with either of them. The performance of the fourth RAFT controller was poor.     

Polymerization-induced self-assembly of block copolymer nanoparticles via RAFT non-aqueous dispersion polymerization

There is considerable current interest in polymerization-induced self-assembly (PISA) via reversible addition–fragmentation chain transfer (RAFT) polymerization as a versatile and efficient route to various types of block copolymer nano-objects. Many successful PISA syntheses have been conducted in water using either RAFT aqueous dispersion polymerization or RAFT aqueous emulsion polymerization. In contrast, this review article is focused on the growing number of RAFT PISA formulations developed for non-aqueous media. A wide range of monomers have been utilized for both the stabilizer and core-forming blocks to produce diblock copolymer nanoparticles in either polar or non-polar media (including supercritical CO₂ and ionic liquids) via RAFT dispersion polymerization. Such nanoparticles possess spherical, worm-like or vesicular morphologies, often with controllable size and functionality. Detailed characterization of such sterically stabilized diblock copolymer dispersions provides important insights into the various morphological transformations that can occur both during the PISA synthesis and also on subsequent exposure to a suitable external stimulus (e.g. temperature).     

Surface patterning of micron-sized aluminum flakes by seeded dispersion polymerization: Towards waterborne colored pigments by gold nanoparticles adsorption

The concomitant polymer embedding and surface patterning of micron-sized aluminum flakes by dispersion polymerization of styrene is reported. The aluminum pigments coated by a thin silica layer were modified by trimethoxysilylpropyl acrylate grafting (TMSPA-grafted Al@silica). Dispersion polymerization of styrene was further performed in alcoholic or hydro-alcoholic media in the presence of the organically modified TMSPA-grafted Al@silica pigments as a seed and poly(N-vinylpyrrolidone) (PVP) as a steric stabilizer to lead to hybrid aluminum flakes. The weight fraction of polymer onto the aluminum pigment surface was tuned from 14 to 29 wt.% by the hydro-alcoholic continuous phase composition and from 19 to 38 wt.% by the initiator content. The chemical nature of the initiator plays an important role in the surface patterning of the hybrid aluminum flakes as 2,2′-azobis(isobutyronitrile) initiator provided polymer nodules, i.e. due to a pattern of polymer protrusions with nodules shape at the surface of the inorganic substrate, while a flat and continuous polymer layer was obtained with 2,2′-azobis(2-amidinopropane) dihydrochloride initiator. Moreover, the introduction of ammonium groups at the surface of the polymer particles was possible by the delayed addition of a cationic acrylate co-monomer ([2-(acryloyloxy)ethyl]-trimethylammonium chloride) rendering the pigments dispersible in water. Finally, gold nanoparticles were successfully deposited onto the micro-patterned aluminum flakes to open the way towards new colored aluminum pigments. Both location of cationic ammonium function in the polymer nodule and nature of gold nanoparticles ligands were investigated with the aim to evaluate the amount of adsorbed gold nanoparticles. The final hierarchically patterned aluminum pigments dispersed in water exhibited an intense purple color.     

Seeded dispersion polymerization

Micron‐size poly(n‐butyl acrylate) (PBA) and polystyrene (PSt) particles were used as seed in second‐stage seeded dispersion polymerizations. The effects of various polymerization parameters on the morphology of the structured particles resulting from the second‐stage seeded dispersion polymerizations were studied, and a series of uniform micron‐size structured particles was successfully prepared. In addition to the polymerization medium, the type of stabilizer (i.e., the molecular weight of the polyvinylpyrrolidone [PVP] stabilizer) that was used in the seed preparation and the subsequent seeded dispersion polymerization was found to be important. The final outcome of a seeded dispersion polymerization, that is, the morphology of the structured particles and the formation of secondary particles, was found to be primarily governed by thermodynamic factors. It was also found that the latex particles in these dispersion systems are virtually stabilized by the small amount of grafted PVP molecules. The dispersions maintain colloidal stability after repeated washing of the particles, which removes all of the soluble PVP. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2710–2720, 2002     

Preparation and characterization of titanium dioxide core and polymer shell hybrid composite particles prepared by two-step dispersion polymerization

Titanium dioxide core and polymer shell composite poly (styrene-co-divinylbenzene)-methacrylic acid [P (St-co-DVB)-MAA]] particles were prepared by two-step dispersion polymerization. Fourier transform IR spectroscopy and elemental analysis were used to measure the content of methacrylic acid in composites particles. X-ray measurement photoelectron spectroscopy (XPS) measurements indicated the presence of an MAA unit on the surface of the composite particles. The combined results of the elemental analysis and the XPS measurements showed that the copolymer on the surface of poly (St-co-DVB)-MAA composite particles was rich in MAA compared with that in the interior of the composite particles. Field-emission scanning electron microscopy (FE-SEM) was used to study the morphology characterization. The composite particles produced showing good spectral reflectance compare with bare TiO₂. TGA results indicated that the encapsulation efficiency and estimated density of composite particles. Encapsulation of TiO₂ was up to 87.4% and the density was ranged from 1.78 to 2.06 g/cm³. Estimated density of the composite particles is suitable to 1.73 g/cm³, due to density matching with suspending fluid.     

Synthesis of latex particles by ring-opening metathesis polymerization

This paper reviews the recent discoveries that were achieved by our group in the field of particle synthesis via ring-opening metathesis polymerization (ROMP). Both polynorbornene and polybutadiene-based particles were prepared by the respective ROMP of norbornene and cyclooctadiene initiated by (PCy₃)₂Cl₂RuCHPh using several methods of polymerization in dispersed media, such as dispersion, suspension and miniemulsion. Depending upon the process implemented, particles size was found to range from 300 nm to 20 μm.     

Dispersion polymerization of styrene in CO2-expanded ethanol

Dispersion polymerization of styrene has been performed in CO₂-expanded ethanol at ≤9 MPa and 70 °C using PVP as stabilizer. The polymerizations proceeded with good colloidal stability, resulting in spherical particles of diameters of ∼2 μm. Pressurization with CO₂ leads to an increase in particle size (∼1 μm in the corresponding CO₂-free system), and a decrease in both polymerization rate and molecular weight. The main effect of CO₂ is proposed to be its influence on the partitioning of monomer between the continuous and the particle phase–the results indicate that CO₂-pressurization causes a reduction in monomer concentration in the particles. Overall, the results are consistent with literature data on the effects of the polarity of the continuous phase in dispersion polymerization of styrene in alcohols and alcohol/water mixtures.     

聚氨酯预聚体——可聚合乳化剂用于MMA乳液聚合动力学及聚合稳定性研究

将合成的聚氨酯预聚体可聚合乳化剂（APUA）用于甲基丙烯酸甲酯（MMA）乳液聚合体系中，研究了不同引发剂体系和乳化剂对聚合体系的聚合稳定性和动力学影响，并与SDS乳化剂进行了对照。结果显示，用油溶性引发剂（AIBN）和水溶性引发剂（K2S2O8）都能引发该体系的聚合反应，而且聚合速率和转化率都相当高。用AIBN作引发剂时，APUA和SDS两种乳化所对应的动力学曲线不同，但APUA比SDS的聚合速率大得多，其分别类似于微乳液聚合（miniemulsion)和微悬浮聚合（microsuspention)，且聚合过程中会产生一定的凝胶。当乳化剂APUA用量适合时，凝胶量极少，聚合体系是稳定的；而当用K2S2O8作引发剂时，两乳化剂对应动力学曲线相似，聚合过程不产生凝胶。但当引发剂用量较大并以APUA作乳化剂时，在聚合过程中会出现全部粒子突然粗化现象，而以SDS作乳化剂没有出现这一现象。     

Monodisperse magnetic composite poly(glycidyl methacrylate)/La0.75Sr0.25MnO3 microspheres by the dispersion polymerization

Monodisperse magnetic poly(glycidyl methacrylate) microspheres were prepared by dispersion polymerization of glycidyl methacrylate in cyclohexane in the presence of La_0.75Sr_0.25MnO₃ nanoparticles, surface of which was modified with penta(propylene glycol) methacrylate phosphate (PPGMAP). However, only agglomerates were formed by the dispersion polymerization in toluene. Sterical stabilizer was poly(styrene-block-hydrogenated butadiene-block-styrene) and initiators investigated were 2,2′-azobisisobutyronitrile (AIBN) and 4,4′-azobis(4-cyanovaleric acid) (ACVA). Effects of initiators and other reaction conditions on properties of the composite microspheres were evaluated. The phase composition of the magnetic polymer composite microspheres and the size of magnetic cores were determined by X-ray powder diffraction. The characterization was completed by magnetization measurements, atomic absorption spectroscopy, TEM, SEM and ATR FTIR spectroscopy.     

Kinetics and mechanism of seeded dispersion polymerization

The kinetics and mechanism of seeded dispersion polymerization of methyl methacrylate (MMA) was studied by applying both micron and submicron PMMA seeds. Using a 1.7 μm PMMA seed (N_p = 1 × 10¹²/L) and a monomer polymer ratio (M/P) of 28/1, secondary nucleation was found to occur and the number of new particles exceeded that produced in a parallel ab initio dispersion polymerization. This was explained by the paradoxical initiator concentration effect seen in dispersion polymerizations where the number of particles decreases with increasing initiator concentration. In contrast, using 194 nm (N_p = 26 × 10¹²/L; M/P = 833/1) and 317 nm (N_p = 5.6 × 10¹²/L; M/P = 714/1) submicron seeds, it was found that the final particle number was similar to (or less in a few cases) the initial seed number over a relatively wide range of initiator concentrations. With increasing initiator concentration, the initial reaction rate increased but the maximum reaction rate decreased slightly. This was explained by increased radical termination particularly in unstable nuclei, leading to a reduced radical entry rate. The reaction rate was found to be moderately dependent on the number of seed particles, but was independent of the seed surface area. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A kinetic study on MMA bulk radical polymerization

The nonequalities of the reactivity and the micro-environment of propagating radicals has been found in MMA bulk polymerization. This nonequal reactivity results from varying rates of diffusion in different micro-surroundings. The decrease of radical reactivity is caused by the trapping of the segments around the propagating radical.     

Living radical dispersion photopolymerization of styrene by a reversible addition-fragmentation chain transfer (RAFT) agent

In this study, an addition-fragmentation chain transfer agent bearing dithioester group is synthesized and applied to conventional dispersion photopolymerization of styrene in ethanol medium in the presence of poly(N-vinylpyrrolidone) stabilizer with varying amounts of the RAFT agent and optionally with conventional initiator, azobisisobutyronitril (AIBN) at various temperatures. Monomer conversion, molecular weight evolution, polydispersity index (PDI), and final particle sizes are measured. The PDI of the formed polymer is between 1.5 and 2.5 in the presence of RAFT agent. Higher concentration of RAFT agent or elevated temperature leads to the acceleration of the polymerization rate resulting in fast conversion, and reducing molecular weight and PDI. Stable polystyrene beads above 1 μm in diameter are successfully prepared by means of RAFT method applied in dispersion polymerization. The weight average particle sizes are between 1.08 and 2.04 μm, and the uniformity (D_w/D_n) is ranged from 1.26 to 2.51.